The present invention relates to a high voltage semiconductor device, and more particularly, to a lateral double doped metal oxide semiconductor (LDMOS) transistor capable of reducing electric field concentration in a source region.
High voltage lateral double doped metal oxide semiconductor (LDMOS) transistors have been widely used as switching devices for control, logic and electric power. The LDMOS transistor is typically designed to have a high breakdown voltage and a low on-resistance so that the LDMOS transistor can sustain high voltages, conduct high currents, have low power consumption, and improved switching characteristics. In a high-voltage semiconductor device, the curvature radius of a source or drain region has a close relation with the breakdown voltage characteristic of a device. Particularly, in some LDMOS transistors, the electric field is typically high at the tip portion of a source region having a relatively small curvature radius, which can result in reduced breakdown voltage.
FIGS. 1A through 1C show simulation results of the electric field concentration around the source tip 10t of a source portion 10 for three devices having source tips 10t with different radius of curvatures r, r′ and r″. FIG. 1D shows an Id-Vd graph where the I-V curve marked as “A” corresponds to the curvature radius r of 8.5 μm (FIG. 1A), the I-V curve marked as “B” corresponds to the curvature radius r′ of 18.5 μm (FIG. 1B), and the I-V curve marked as “C” correspond to the curvature radius r″ of 28.5 μm (FIG. 1C) As can be seen, as the radius of the tip 10t of the source portion 10 is reduced, the electric field concentration at the tip portion 10t is increased, and thus the breakdown voltage of the device is reduced.
Thus, in order to obtain a high breakdown voltage due to the high electric field concentration at the tip 10t of the source portion 10, the curvature radius of the tip 10t of the source portion 10 needs to be increased. However, increasing the curvature radius of the tip 10t of the source portion 10 disadvantageously increases the die size.
Thus there is a need for a technique which, among other advantages and features, enables reducing the curvature radius of the tip of the source without adversely impacting the device breakdown voltage.